The invention relates to a diaphragm drive apparatus with diaphragm preselector ring for objectives preferably of long or variable focal length, with pressure release diaphragm actuatable automatically by a plunger, where the diaphragm closure movement is transmitted through a preferably two-armed lever arm in force-engaging manner to a blade drive ring, the plunger having a surface or cam face inclined in relation to its direction of movement, with an adjoining surface without rise.
In the diaphragm actuation of objectives with automatic pressure release diaphragm, firstly during a prescribed stroke of the plunger the diaphragm closes to the preselected value, then the plunger must be capable of being pressed in further to a specific amount which is given by the shutter release point of the camera (over-stroke). Therefore in the diaphragm drive mechanism of the known objectives with automatic pressure release diaphragm, usually a spring deflector is arranged which becomes effective when the diaphragm abutment is reached.
The maximum counter-force of the spring deflector is limited by the permissible press-in force on the actuating plunger. The force of the diaphragm opening spring must be selected so great that it reliably brings the diaphragm into the open position on liberation of the plunger. The closing of the diaphragm in the case of jerky pressing in of the actuating plunger from the camera is thus effected in these known objectives by the difference between the spring force of the spring deflector and the force of the opening spring.
In an automatic preselector diaphragm having a diaphragm actuation lever rotatably mounted on a movably and spring loaded carrier, it has also already been proposed to make the rest position of this carrier adjustable, in dependence upon a control cam face arranged on the diaphragm ring, so that in the gear train between actuating plunger and diaphragm actuating lever a comparatively large idle stroke occurs which corresponds to the pivot distance, suppressed in each case by the diaphragm abutment cam, of the diaphragm actuating lever. Thus in fact the travel of the plunger which in each case is not necessary for the closing of the diaphragm is converted into an idle stroke. During the over-stroke of the actuating plunger in this case however the spring loading of the movable carrier acts as spring deflector and again diaphragm closure on jerky pressing in of the actuating plunger takes place only due to the differential force between diaphragm opening spring and spring deflector force. It is here disadvantageous that after the reaching of the preselected diaphragm aperture the actuating force rises suddenly in each case due to the response of the spring deflector.
In several other known objectives the diaphragm is held in the preselected position by a closer spring when the plunger is pressed in, and on lilberation of the plunger the diaphragm is opened against the force of the closer spring by a powerful spring arranged on the actuating plunger or on the lever driven by the latter. The closing of the diaphragm takes place thus due to the action of a closer spring. However its force must likewise be less than the permissible actuating force on the plunger, because the opening of the diaphragm is effected by the force difference of the two springs.
As regards the configuration of the plunger which effects the diaphragm closing movement either directly or through a lever linkage to the diaphragm ring, it is known that the plunger comprises a surface inclined in relation to its direction of movement or a cam face with a surface without rise adjoining this, which latter surface can serve for the idle stroke or overstroke of the plunger. The guide member for the diaphragm setting mechanism rests in force-engaging manner by means of a spring on the cam face of the plunger.
As is known, in the use of objectives with automatic pressure release diaphragm on modern cameras, specific closure time requirements are important. The diaphragm must follow the jerky press-in movement of the plunger as far as the value preselected in each case within a specific time, and the recoil phenomena caused by the diaphragm abutment must likewise have died away. These recoil phenomena are increased by the fact that the gearing part to be accelerated is halted only in force-engaging manner in its end position. Therefore observations on the closure time conditions in known objectives of long focal length always show inconsistencies on account of the larger diaphragm thereof so that diameters considerably greater masses have to be accelerated, which is problematical within the prescribed force limits. There is also the fact that on reaching of the diaphragm abutment these greater masses must be suddenly braked and thus recoil phenomena occur to a greater extent, which cannot be caused to die away sufficiently rapidly.
It is the object of the invention to provide a drive apparatus in which the available plunger press-in force is largely exploited for the closure of the diaphragm and in which at the same time recoils are largely suppressed.